Process for the centrifugal separation of sludge-containing liquids



Dec. 5, 1950. N E SVENSJO 2,532,792

PROCESS FOR fHEfCENTRIFUGAL. SEPARATION OF SLUDGE CONTAINING LIQUIDSFiled April 13, 11946v 2 Sheets-Sheet l FLOW REGULflTOR l i/070C541. 3

ilaw REGUtfiTOR 7 L 5 Dec. 5, 1950 N E svENsJo 2,532,792

PROCESS FOR THE CENTRIFUGAL SEPARATION OF SLUDGE CONTAINING LIQUIDS 2Sheets-Sheet 2 Filed April 13, 1946 I/VVE/VI'UK A zZ; 41 3? 31/5875named Dec. 5, 1950 PROCESS FOR THE CENTRIFUGAL SEPARA- TION OFSLUDGE-CONTAINING LIQUIDS Nils Edvin Svensjii, Nockeby, Sweden, assignorto Aktiebolaget Separator, Stockholm, Sweden, a corporation of SwedenApplication April 13, 1946, Serial N0. 661,997 In Sweden April 18, 19452 Claims. (01. 233-14) This invention relates to a process for thecentrifugal purification of sludge-containing liquids. The invention isparticularly applicable to the removal of solid particles from a liquidin which they are in suspension, by means of a centrifugal separator ofthe type in which the bowl is provided at its peripherywiti'isutletopenings through which the solid particles leave the bowltogether with a certain quantity ofv carrier liquid.

The sludge-discharge openings should be large enough to allow solidparticles to freely pass therethrough. However, if the content of solidparticles is small, a concentrate very poor in sludge is dischargedthrough the openings. This is usually undesirable, as, for example, ifthe liquid in the concentrate must be removed later on by evaporat on.

To avoid this excessive discharge of carrier liquid with the sludge, ithas been proposed to return varying amounts of the concentrate into thebowl, whereby is obtained an arb trary increase in the sludge content ofthe concentrate in the bowl. However, the concentration must not be toohigh, since this causes the openings in the bowl to become clogged.

In practice it has been necessary to work at rather low concentrations,it being diflicult to judge at what sludge-content of the concentrateclogging takes place. As the sludge-content of the unseparated liquidoften varies considerably and as a discharge opening or several of thedischarge openings may be clogged by separate large particles which haveentered the bowl, the maintenance of a desirable sludge content in theseparator is diflicult. In practice it has been found necessary to workwell below the maximum capacity of the separator.

I have found that a too high sludge content of the concentrate is notthe sole cause of the clogging. The size and form of the particles arealso contributory clogging factors.

I have found that viscosity is the determining factor influencing thedanger of clogging.

- Viscosity is dependent on many factors, such as the sludge content,the temperature of the liquid to be separated and the size and form ofthe particles. By determining the vscosity of the sludge-containingconcentrate, discharged from the bowl, and by regulating, in accordancewith such viscosity determination, either the percentage of thedischarged sludge-containing concentrate returned to the bowl, or, lesspreferably, the rate of feed of the mixture fed to the bowl, the viscosty of the concentrate may be main. tained substantially constant.

The accompanying drawing shows an arrangement in which theabove-mentioned method of regulation is employed. Fig. 1 showsdiagrammatically the whole installation, whilst Fig. 2 is a detailedillustration. of one form of the regulating device. In the drawing, Idesignates the centrifugal separator bowl or locus of centrifugal force,2 the feedpipe for the mixture to be separated, 3 the discharge-pipe forthe separated liquid free from sludge and 4 the discharge pipe for theseparated sludge-containing concentrate. Part of the concentrate is fedback to the bowlthrough the ppe 5 to increase the viscosity of theconcentrate in the bowl. The remaining concentrate leaves the system viathe outlet pipe 6.

According to the invention, the viscosity of the sludge concentratedischarged through pipe 4 is determined, and the quantity of concentrateled back to the bowl through pipe 5 is varied in accordance with anydeparture of the determined viscosity from a desired viscosity. That is,if said determined viscosity is above standard, the percentage of thedischarged concentrate that is returned to the bowl is reduced. If saiddetermined viscosity is below standard, the percentage of the dischargedconcentrate that is returned to the bowl is increased. The effect is tomaintain the viscosity of the discharged concentrate at a substantiallyconstant value.

The drawing discloses means whereby the above descr bed operation may beeflected automatically. Interposed in the pipe 4 is a continuous flowviscosimeter 1, which operates through a connection Ia to control aquantity-regulating device 8 in the pipe 6. If, for instance, theviscosity as measured by the viscosimeter rises above a predeterminedvalue, the device 8 is actuated from the viscosimeter to increase theliquid flow through the pipe 6 and so to reduce the flow through thepipe 5 until theviscosity is reduced to the desired value. Thepredetermined viscosity should be as high as is possible consistent withavoidance of clogging of the sludge discharge openings.

The quantity of sludge in the concentrate may vary and also thetemperature and the form and size of the particles, but the concentrateshould always have as high a content of solid particles as is consistentwith the maintenance of a free dscharge from the bowl.

It is important that the quantity-regulating device 8 should control theliquid-flow through the pipe 6. If one of the openings in the bowlshould get clogged by a large particle, thus re- (lim ng the total flowthrough the pipe 4, the return-flow through the pipe I will be reducedimmediately, whereby a temporary increase of the concentration in thebowl is prevented. The quantity regulator 8 is so designed that itscapacity is not influenced in a substantial degree by the quantity ofliquid flowing out through the openings per unit of time. It maycomprise, for example, as shown in Fig. 2, a gear-pump II, or other typeof pump, whose speed is regulated by the viscosimeter 1. Such aviscosimeter may consist of a rotatably supported drum l2, connected tothe pipe 4 by a flexible inlet I3 and a flexible outlet it. Thus theconcentrate discharged from the separator l flows through the drum. Inthe drum there is enclosed a rotatable element ll, which is driven at aconstant speed by an electric motor IS. The rotary motion of the drum i2is, by means of two gears l1 and II, transmitted to the movablecontact-arm l 9 of a variable rheostat. A small or great part of theresistance of the rheostat can be switched in the circuit of anelectric'motor 2|, which drives the pump I l. A spring 22 tends tomaintain a certain part of the resistance 20 in the circuit. The torsionmoment transmitted from the rotating element ii to the drum l2 by theintermediary of the flowing concentrate, counteracts this tendency ofthe spring.

The concentrate flowing through the drum i2 will transmit a torsionmoment to the drum from the constant speed rotating element IS, themagnitude of the transmitted torsion moment depending upon the viscosityof the concentrate in the drum. Thus, when the viscosity increases, areater torsion moment will be transmitted to the drum l2 so ,that thelatter, operating through the gearing l'l-l8, will act against thespring 22 and cut out additional resistance 20 of the rheostat. As aresult, the motor 2| will drive pump II at a higher speed and therebyreduce the rate of flow of concentrate through the return pipe 5 to thecentrifuge. Conversely, when the viscosity of the concentrate decreases,the torsion moment transmitted from rotating element ii to the drum i 2will decrease and allow the spring 22 to cut in additional resistance 20of the rheostat. The speed of the pump II will then decrease and causean increase in the rate of flow of concentrate through return line 5 tothe centrifuge.

The viscosimeter may be made to control a quantity-regulating device inthe pipe 2, but this arrangement is not as desirable as the control ofthe flow from pipe 6, since the separator would not then operate at itshighest separating capacity.

An arrangement according to the above may comprise a photocell 9, in thepurified liquid discharge pipe 3, controlling a quantity-regulatingdevice Ill in the feed-pipe 2 in such a way that the feed to theseparator is so regulated that a predetermined degree of purity in theliquid is insured. The separator may be operated at capacity and at thesame time badly separated liquid is prevented from passing from thesystem if some opening in the bowl is clogged.

The photocell 9, of course, will respond to changes in the lighttransmitting ability of the purified liquid discharging through pipe I,and operates through a suitable connection 9a to control the flowregulator I, so that the rate of feed through pipe 2 is reduced when thecontent of sludge particles in the purified liquid (pipe 8) increases,and vice versa. It will be apparent that if, at a given throughput ratefor the separator, the viscosity of the sludge becomes $00 4high,thelimitoi'theseparatingabilityofflie separator may be exceeded,whereby part of the sludge from the separator will accompany the liquidcomponent flowing through discharge pipe 2. Accordingly, under thiscondition, not only will the viscosity of the outfiowing sludge in pipe4 increase, but also the light transmitting ability of the liquidcomponent (pipe 2) will decrease, as seen" by photocell 8, due to suchintermixing of sludge with the liquid component. The arrangement of thephotocell and flow regulator II is illustrated only schematically, assuch arrangements per se are well known in the art. As an example of aconventional arrangement, the pipe I may have a transparent sectionlocated between a light source and the photo-sensitive element, so

that the light must pass through the transparent section (and the liquidflowing in it) to reach the photo-sensitive element. Theelectric'current changes in the latter, which are generally amplified,may be used to actuate a servomotor operating through connection so tocontrol the flow regulator it, which may be a valve or a variable speedpump.

A method such as described will separate a concentrate with a varyingsludge content, which, however, is at all moments as high in sludgecontent as possible when either the separator is operating with a freefiow through the bowl or is partially clogged. A partial clogging of thedischarge effects only a reduction of the capacity, but does notincrease the viscosity of the concentrate or involve any'deteriorationin quality of the separated liquid. Alternatively, the feed need notnormally be as low as when the risk of a partial clogging has to beconsidered.

What I claimand desire to protect by Letters Patent is:

1. In an apparatus for continuously purifying a sludge-containingliquid, comprising a centrifuge having a separating chamber with anoutlet for discharging liquid bearing a concentration of separatedsludge and also having an outlet lor discharging the purified separatedliquid, a discharge pipe connected to said first outlet, and a returnpipe leading from said first pipe for returning to the centrifuge partof said liquid bearing the sludge concentrate, the combination of aviscosimeter connected to the discharge pipe at a location between saidfirst outlet and said return pipe, flow regulating means in thedischarge pipe and located beyond the return pipe with respect to saidfirst outlet, and an operative connection between the viscosimeter andsaidv regulating means for operating the regulating means in accordancewith variations in the indications of the viscosimeter, whereby the rateof return of sludge concentrate to the centrifuge is varied.

2. In the process of separating solids from a mixture of solids andliquid, by continuously feeding the mixture into a locus of centrifugalforce to separate the same into a purified liquid and asludge-containing concentrate, continuou.

1y discharging said separated liquid and concenin reasing the rate orflow at said region in recentrate discharge, and decreasing the rate offlow at said region in response to a decrease in the viscosity of theconcentrate discharge, whereby the viscosity of the concentratedischarge is maintained approximately constant, and in which saidchanges in the rate of flow at said region of the concentrate dischargestream are effected in response to variations in the viscosity asdetermined at a part of said last stream between said locus and thepoint of withdrawal of the return concentrate.

NILS EDVIN svENsJ6.

REFERENCES CITED The following references are of record in the file ofthis patent:

Number Number 6 UNITED STATES PATENTS Name Date Ericsson Feb. 9, 1909Sharples July 3, 1917 MacKaye Oct. 2, 1923 John Dec. 26, 1939 Hagy Jan.25, 1944 Cram Jan. 1, 1946 FOREIGN PATENTS Country Date Great BritainOct. 17, 1929 Great Britain May 8, 1935 Germany July 5, 1939 SwedenSept. 8; 1942

